Ferrite magnet powder and magnet using the magnet powder, and method for preparing them

ABSTRACT

The ferrite magnet powder of the present invention is magnet powder having, as the major phase, a La—Co magnetoplumbite ferrite where La and Co are substituted for Sr and Fe, respectively, represented by (1−x)SrO.(x/2)La 2 O 3 .(n−y/2)Fe 2 O 3 .yMO wherein x, y, and n represent mole ratios and satisfy 0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0, where x&gt;y.

TECHNICAL FIELD

The present invention relates to ferrite magnet powder, a magnet usingthe ferrite magnet powder, and methods for manufacturing the same.

BACKGROUND ART

The ferrite is a generic name for compounds produced from oxides ofdivalent cationic metals and trivalent iron. Ferrite magnets have foundvarious applications in motors, generators, and the like. As materialsfor the ferrite magnets, widely used are magnetoplumbitehexagonal-structured Sr ferrites (SrFe₁₂O₁₉) and Ba ferrites(BaFe₁₂O₁₉). These ferrites are produced at comparatively low cost by apowder metallurgical method using iron oxides and carbonates ofstrontium (Sr), barium (Ba), and the like.

The basic composition of the magnetoplumbite ferrites is generallyrepresented by a chemical formula “MO.nFe₂O₃” where element M is a metalthat is to serve as divalent cations, selected from Sr, Ba, Pb, Ni, andthe like. Iron ions (Fe³⁺) at respective sites of the ferrite, whichhave a spin magnetic moment, are coupled by superexchange interactionvia intermediate oxygen ions (O²⁻). The magnetic moment of Fe³⁺ ions attheir sites are oriented “upward” or “downward” along the c axis.Because the number of sites having an “upward” magnetic moment isdifferent from the number of sites having a “downward” magnetic moment,the ferrite exhibits ferromagnetism (as a ferrimagnet) as the entirecrystal.

It is known that, among the magnetic performance of the magnetoplumbiteferrite magnets, the residual magnetic flux density (B_(r)) can beenhanced by improving the Is of crystals and increasing the density of asintered body and the degree of orientation of the crystals. Also knownis that the coercive force (H_(cj)) can be enhanced by increasing therate of existence of single-domain crystals. However, an attempt of byincreasing the density of the sintered body to enhance the residual fluxdensity (B_(r)) will facilitate crystal growth, resulting in reducingthe coercive force (H_(cj)). In reverse, an attempt of enhancing thecoercive force by controlling the size of crystal grains by addition ofAl₂O₃ and the like will reduce the density of the sintered body,resulting in reducing the residual flux density. Various studies weremade on the compositions, additives, and production conditions offerrites for the purpose of enhancing the magnetic properties of theferrite magnets. However, it was found difficult to develop a ferritemagnet enhanced both in residual flux density and coercive force.

The applicant of the present invention developed a ferrite magnet ofwhich the coercive force was enhanced without reduction in residual fluxdensity by adding Co to a raw material (Japanese Patent ExaminedPublication Nos. 4-40843 and 5-42128).

After the above development, there was proposed a ferrite magnet ofwhich the saturation magnetization (σ_(s)) was enhanced by substitutingZn and La for Fe and Sr, respectively (Japanese Laid-Open PublicationNos. 9-115715 and 10-149910). A ferrite magnet has relatively lowsaturation magnetization because it is a ferrimagnet in which themagnetic moments of Fe³⁺ ions orient in opposite directions depending onthe sites as described above. According to the above publications,however, the “downward” magnetic moments can be reduced by placing ionshaving a magnetic moment smaller than the magnetic moment of Fe inspecific sites of Fe ions, to thereby enhance the saturationmagnetization σ_(s). The publications also describe examples using Ndand Pr in place of La, and Mn, Co, and Ni in place of Zn.

The Abstracts of the Magnetics Society of Japan Annual Meeting(distributed on Sep. 20, 1998) discloses ferrite magnets of which boththe coercive force (Hcj) and the saturation magnetization (σ_(s)) areenhanced by use of La- and Co-added compoundsSr_(1−x)La_(x)Co_(x).Fe_(12−x)O₁₉.

The above ferrite magnets are still insufficient in improvement of boththe coercive force and the saturation magnetization.

The above-mentioned abstracts (distributed on Sep. 20, 1998) report thatthe coercive force can be improved to some extent by substituting Co, inplace of Zn, for Fe, but fail to describe the cause of this improvement.In addition, the degrees of the improvement of the coercive force andthe residual flux density are considered insufficient.

DISCLOSURE OF THE INVENTION

In view of the above, the main object of the present invention isproviding ferrite magnet powder enhanced both in saturationmagnetization and coercive force, and magnet using such magnet powder.The magnet powder of the present invention is magnet powder having aferrite major phase represented by (1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO(where A denotes one or two kinds of metal selected from Sr and Ba, Rdenotes a rare earth element necessarily including La, and M denotes adivalent metal necessarily including Co), wherein x, y, and n representmole ratios and satisfy 0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02,and 5.2≦n≦6.0, where x>y.

The bond magnet of the present invention includes the magnet powderdescribed above, and the sintered magnet of the present invention ismade of the magnet powder described above.

The method for manufacturing magnet powder of the present inventionincludes the steps of: preparing raw material mixed powder of SrCO₃powder and Fe₂O₃ powder with addition of powder of oxides of La and Co;calcinating the raw material mixed powder to form a ferrite calcinatedproduct as magnet powder having a ferrite major phase represented by(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes one or two kinds ofmetal selected from Sr and Ba, R denotes a rare earth elementnecessarily including La, and M denotes a divalent metal necessarilyincluding Co), wherein x, y, and n represent mole ratios and satisfy0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0, where x>y;and pulverizing the calcinated product.

The method for manufacturing a magnet of the present invention includesthe steps of: preparing raw material mixed powder of SrCO₃ powder andFe₂O₃ powder with addition of powder of oxides of La and Co; calcinatingthe raw material mixed powder to form a ferrite calcinated product asmagnet powder having a ferrite major phase represented by(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes one or two kinds ofmetal selected from Sr and Ba, R denotes a rare earth elementnecessarily including La, and M denotes a divalent metal necessarilyincluding Co), wherein x, y, and n represent mole ratios and satisfy0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0, where x>y;pulverizing the calcinated product to form ferrite magnet powder; andcompacting and sintering the ferrite magnet powder.

The method for manufacturing a magnet of the present invention includesthe steps of: preparing raw material mixed powder of SrCO₃ powder andFe₂O₃ powder with addition of powder of oxides of La and Co; calcinatingthe raw material mixed powder to form a ferrite calcinated product asmagnet powder having a ferrite major phase represented by(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes one or two kinds ofmetal selected from Sr and Ba, R denotes a rare earth elementnecessarily including La, and M denotes a divalent metal necessarilyincluding Co), wherein x, y, and n represent mole ratios and satisfy0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0, where x>y;pulverizing the calcinated product to form ferrite magnet powder; andforming a bond magnet from the ferrite magnet powder.

The value of n is preferably in the range of 5.4≦n≦5.7, and x/y ispreferably in the range of 1.1 to 1.3.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a graph showing the relationship between the mole ratio nrepresenting the composition of a product and the residual flux densityB_(r), and FIG. 1(b) is a graph showing the relationship between themole ratio n representing the composition of the product and thecoercive force H_(cj).

BEST MODE FOR CARRYING OUT THE INVENTION

The magnetic powder of the present invention includes, as a major phase,a ferrite represented by (1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO where Adenotes one or two kinds of metal selected from Sr and Ba, R denotes arare earth element necessarily including La, and M is a divalent metalnecessarily including Co.

In the above formula, x, y, and n represent mole ratios, and satisfy therelations 0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0,respectively, where x>y. The value n more preferably satisfies therelation 5.4≦n≦5.7 as will be described later. Also, x/y is preferablyin the range between 1.1 and 1.3, and more preferably x/y=1.2.

In the present invention, in order to substitute M²⁺ for Fe³⁺ of themagnetoplumbite ferrite despite of difference in valence, the element Ais used to substitute for part of Fe to compensate the difference invalence. It was conventionally considered that the substitute amounts xand y should preferably be substantially equal to each other. In thepresent invention, however, x/y is set in the range between 1.1 and 1.3as described above.

The method for manufacturing magnet powder of the present invention willbe described.

Powder of an oxide of the element A and powder of Fe₂O₃ (α ferric oxide)are mixed at a mole ratio in the range of 1:5.2 to 1:6.0. During thismixture, an oxide of the element R, an oxide of the element M, and thelike are added to the raw material powder. The primary particle sizes ofthe respective powders are about 0.8 μm for SrCO₃, about 0.5 μm forFe₂O₃ powder, about 1.0 9 m for La₂O₃, and about 1.0 μm for CoO, forexample.

The elements R and M is added to the raw material powder preferably inthe form of powder of the respective oxides as described above.Alternatively, it may be added in the form of powder of compounds otherthan oxides (for example, carbonates, hydroxides, nitrates, and thelike).

Other compounds including SiO₂, CaO, CaCO₃, SrCO₃, Al₂O₃, Cr₂O₃, and thelike may also be added as required in the amount of about 1% by weight.

The mixed raw material powder is then heated to a temperature of 1300 to1400° C. in the atmosphere by use of a rotary kiln or the like tosubject the mixture to solid-phase reaction to thereby produce amagnetoplumbite ferrite compound. This process is called “calcination”and the resultant compound is called a “calcinated product”. Thecalcination time is preferably 15 minutes to 3 hours.

The calcinated product obtained in the calcination process includes, asa major phase, a magnetoplumbite ferrite represented by the formula

(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃ .yMO

where A denotes one or two kinds of metal selected from Sr and Ba, Rdenotes a rare earth element necessarily including La, and M denotes adivalent metal necessarily including Co. The average particle size ofthe calcinated product is in the range of 0.5 to 5 μm.

In the above formula, x, y, and n represent mole ratios, and the rawmaterial powders had been weighed so as to satisfy the relations,0.22−0.02≦x≦0.22+0.02, 0.18−0.02≦y≦0.18+0.02, and 5.2≦n≦6.0,respectively.

The calcinated product is pulverized or deagglomerated to obtain magnetpowder having an average particle size of 0.6 to 1.0 μm. The calcinatedproduct is first roughly pulverized with a roller mill or a rod mill andthen finely pulverized with a wet ball mill or a wet attritor. In thispulverization process, which uses a steel ball placed in an aqueoussolvent, a trace amount of Fe₂O₃ is inevitably mixed in the powder bywear of the steel ball. If Fe is excessively contained, the magneticproperties are deteriorated. Therefore, according to the presentinvention, in expectation of entering of Fe₂O₃, the rare earth element(La and the like) is mixed in an excessive mole amount compared with theequivalent mole amount of the divalent metal M such as Co to be mixed(x>y). The excessive La, as well as Fe, is re-diffused in the crystaland simultaneously diffused in the grain boundary, during sintering. Asa result, good magnetic properties are exhibited as will be describedlater. This effect is exhibited significantly when x/y is in the rangeof 1.1 to 1.3, and is optimized when it is about 1.2.

The finely pulverized raw material obtained by the pulverization processdescribed above is then compacted. Two compaction methods may beadopted. One is a wet compaction method where the finely pulverized rawmaterial is directly put in a die and compacted in the magnetic fieldwhile being dehydrated. The other is a dry compaction method where theraw material is first dried and deagglomerated before being compacted ina die in the magnetic field. The degree of anisotropic orientation ofcrystal grains in the magnetic field is superior in the wet compactionmethod due to difference in aggregation of the finely pulverized rawmaterials. The wet compaction method is therefore suitable inmanufacture of high-performance permanent magnets. The resultant compactis sintered in the atmosphere at 1180 to 1240° C. for the duration ofabout five minutes to about two hours. To attain high density of thesintered product while suppressing crystal growth, a sintering adjuvantsuch as SiO, and CaO is preferably added to the finely pulverized rawmaterial.

The ferrite magnet powder described above may also be mixed withflexible rubber, rigid and light plastic, or the like and solidified toproduce a bond magnet. More specifically, the magnet powder of thepresent invention is mixed and kneaded with a binder and an additive,and then formed. The formation may be made by injection molding,extrusion, roll forming, or the like.

Thus, according to the present invention, provided is a magnetoplumbiteferrite magnet including a substitute element introduced in an optimumcomposition in consideration of the iron component inevitably enteringin the production process.

EXAMPLE 1

First, raw material powder mixed to have a composition of(1−x)SrO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO where x=0.22, y=0.18, and n=5.8was calcinated at 1300° C. for about two hours, and then roughlypulverized with a sample mill to a particle size of about 3 μm or less,to obtain powder of a calcinated raw material.

The calcinated raw material was then finely pulverized by a wet methodwith a ball mill to obtain an average particle size of 0.6 to 0.7 μm.During the pulverization, 0.6 wt % of CaO and 0.45 wt % of SiO₂ wereadded.

The thus-obtained raw material slurry of finely pulverized powder wasspontaneously settled and concentrated to have a solid content of 60 wt.%, and then subjected to compaction in the magnetic field using a diehaving an outer diameter φ36 and a depth of 30 mm.

The resultant compact was sintered at 1220° C. for 30 minutes to producea sintered magnet.

The resultant sintered magnet had the magnetic properties of theresidual flux density Br=0.444 T (4440 G), HCB=4090 Oe (325 kA/m), thecoercive force H_(cj)=355 kA/m (4460 Oe), and the maximum energy product(BH)_(max)=37.9 kJ/m³ (4.76 MGOe).

The analysis results (wt %) of the raw material components are as shownin Table 1 below.

TABLE 1 Fe₂O₃ SrO BaO La₂O₃ CoO CaO SiO₂ Calcinated raw 87.28 7.61 0.223.47 1.33 0.02 0.03 material (pellet) Sintered product 86.66 7.35 0.213.44 1.33 0.60 0.42

While the mole ratio n characterizing the M-type ferrite magnet wassmaller (n=5.77) than the stoichiometric value in the analysis resultsof the calcinated raw material, it increased to a larger value (n=5.90)in the final sintered body. This indicates admixing of Fe₂O₃.

The value n was calculated using the equation, n=(n₁+n₂/2)/(n₃+n₄+2n₅)where n₁ to n₅ are defined as follows.

n₁: Analysis value of Fe₂O₃/Molecular weight of Fe₂O₃

n₂: Analysis value of CoO/Molecular weight of CoO

n₃: Analysis value of SrO/Molecular weight of SrO

n₄: Analysis value of BaO/Molecular weight of BaO

n₅: Analysis value of La₂O₃/Molecular weight of La₂O₃

EXAMPLE 2

A raw material powder mixed to have a composition of(1−x)SrO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO where x=0.22, y=0.18, and n=5.4was calcinated at 1350° C. for about two hours, and then roughlypulverized with a sample mill to a particle size of about 3 μm or less,to obtain powder of a calcinated raw material.

The calcinated raw material was then finely pulverized by a wet methodwith a ball mill to obtain an average particle size of 0.6 to 0.7 μm.During the pulverization, 0.6 wt % of CaO and 0.45 wt % of SiO₂ wereadded.

The thus-obtained raw material slurry of finely pulverized powder wasspontaneously settled and concentrated to have a solid content of 62 wt.%, and then subjected to compaction in the magnetic field using a diehaving an outer diameter φ36 and a depth of 30 mm.

The resultant compact was sintered at 1220° C. for 30 minutes to producea sintered magnet.

The resultant sintered magnet had the magnetic properties of theresidual flux density Br=0.445 T (4450 G), HCB=4110 Oe (327kA/m), thecoercive force H_(cj)=352 kA/m (4420 Oe), and the maximum energy product(BH)_(max)=38.0 kJ/m³ (4.78 MGOe).

The analysis results (wt %) of the raw material components are as shownin Table 2 below.

TABLE 2 Fe₂O₃ SrO BaO La₂O₃ CoO CaO SiO₂ Calcinated raw 86.68 8.23 0.223.47 1.33 0.01 0.03 material (pellet) Sintered product 86.02 7.92 0.213.44 1.32 0.61 0.42

While the mole ratio n characterizing the M-type ferrite magnet wassmaller (n=5.40) than the stoichiometric value in the analysis resultsof the calcinated raw material, it increased to a larger value (n=5.53)in the final sintered body. This indicates admixing of Fe₂O₃. The valuen was calculated using the same equation as that used in Example 1.

Tables 3 and 4 below show the residual flux density B_(r) and thecoercive force H_(cj), respectively, of various magnets having differentx and y values in the composition of(1−x)SrO.(x/2)La₂O₃.(n−y/2)Fe₂O₃.yCoO.

TABLE 3 Mixing proportion of CoO (y) B_(r) (T) 0.135 0.150 0.165 0.1800.195 Mixing proportion 0.18 0.427 0.430 0.432 0.430 0.425 of La₂O₃ 0.200.432 0.433 0.435 0.441 0.435 (x) 0.22 0 0.436 0.442 0.445 0.442 0.24 00.438 0.440 0.442 0.443 0.26 0 0 0.426 0.432 0.439

TABLE 4 Mixing proportion of CoO (y) H_(cj) (kA/m) 0.135 0.150 0.1650.180 0.195 Mixing proportion 0.18 310 314 330 339 326 of La₂O₃ 0.20 322327 350 355 345 (x) 0.22 0 334 354 358 350 0.24 0 334 346 355 357 0.26 00 326 334 341

From the experiments performed for preparing Tables 3 and 4, it wasfound that x>y should preferably beth and that excellent magneticproperties were obtained when x/y was in the range of 1.1 to 1.3. Inparticular, the most excellent magnetic properties were obtained whenx/y=1.2.

FIGS. 1(a) and 1(b) show the relationships between the mole ratio n andthe magnetic properties (the residual flux density B_(r) and thecoercive force H_(cj)).

In consideration of both the residual flux density B_(r) and thecoercive force H_(cj), it is found that preferable magnetic propertiesare obtained when 5.2≦n≦6.0 and more preferable magnetic properties areobtained when 5.4≦n≦5.7.

In the examples described above, Co was substituted for part of Fe ofthe magnetoplumbite ferrite. Alternatively, Co and Zn may be substitutedfor part of Fe, to provide substantially the same effect.

INDUSTRIAL APPLICABILITY

According to the present invention, the proportion of an element addedfor substitution can be optimized, so that both the saturationmagnetization and the coercive force can be improved simultaneously. Asa result, it is possible to manufacture a calcinated product, magneticpowder, and a magnet that are excellent in saturation magnetization andcoercive force.

What is claimed is:
 1. Magnet powder of magnetoplumbite ferrite having aferrite phase represented by (1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where Adenotes on or two kinds of metal selected from Sr and Ba, R denotes arare earth element necessarily including La, and M denotes a divalentmetal necessarily including Co), wherein x, y, and n represent moleratios and satisfy 0.20≦x≦0.24 0.16≦y≦0.20, and 5.2≦n≦6.0, where x>y. 2.Magnet powder of magnetoplumbite ferrite having a ferrite phaserepresented by (1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes on ortwo kinds of metal selected from Sr and Ba, R denotes a rare earthelement necessarily including La, and M denotes a divalent metalnecessarily including Co), wherein x, y, and n represent mole ratios andsatisfy 0.20≦x≦0.24 0.16≦y≦0.20, and 5.4≦n≦5.7, where x>y.
 3. A bondmagnet including the magnet powder according to claim 1 or
 2. 4. Asintered magnet made of the magnet powder according to claim 1 or
 2. 5.A method for manufacturing magnet powder comprising the steps of:preparing raw material mixed powder of SrCO₃ powder and Fe₂O₃ powderwith addition of powder of oxides of La and Co; calcinating the rawmaterial mixed powder to form a ferrite calcinated product as magnetpowder having a ferrite phase represented by(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes on or two kinds ofmetal selected from Sr and Ba, R denotes a rare earth elementnecessarily including La, and M denotes a divalent metal necessarilyincluding Co), wherein x, y, and n represent mole ratios and satisfy0.20≦x≦0.24 3 0.16≦y≦0.20, and 5.2≦n≦6.0, where x>y; pulverizing thecalcinated product.
 6. The method for fabricating magnet powderaccording to claim 5, wherein the value of n satisfies 5.4≦n≦5.7.
 7. Amethod for manufacturing magnet powder comprising the steps of:preparing raw material mixed powder of SrCO₃ powder and Fe₂O₃ powderwith addition of powder of oxides of La and Co; calcinating the rawmaterial mixed powder to form a ferrite calcinated product as magnetpowder having a ferrite phase represented by(1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes on or two kinds ofmetal selected from Sr and Ba, R denotes a rare earth elementnecessarily including La, and M denotes a divalent metal necessarilyincluding Co), wherein x, y, and n represent mole ratios and satisfy0.20≦x≦0.24 0.16≦y≦0.20, and 5.2≦n≦6.0, where x>y; pulverizing thecalcinated product to form ferrite magnet powder; and compacting andsintering the ferrite magnet powder.
 8. A method for manufacturingmagnet powder comprising the steps of: preparing raw material mixedpowder of SrCO₃ powder and Fe₂O₃ powder with addition of powder ofoxides of La and Co; calcinating the raw material mixed powder to form aferrite calcinated product as magnet powder having a ferrite phaserepresented by (1−x)AO.(x/2)R₂O₃.(n−y/2)Fe₂O₃.yMO (where A denotes on ortwo kinds of metal selected from Sr and Ba, R denotes a rare earthelement necessarily including La, and M denotes a divalent metalnecessarily including Co), wherein x, y, and n represent mole ratios andsatisfy 0.20≦x≦0.24 0.16≦y≦0.20, and 5.2≦n≦6.0, where x>y; pulverizingthe calcinated product to form ferrite magnet powder; and forming a bondmagnet from the ferrite magnet powder.
 9. The method for fabricating amagnet according to claim 7 or 8, wherein the value of n satisfies5.4≦n≦5.7.